Conventionally, a technique has been known which detects a terminal malfunction such as abnormal opening of a signal input terminal or abnormal disconnection of a transmission path for an input signal in a signal control device which AD-converts an analog signal input thereto from a sensor and uses the resulting signal for a control arithmetic operation. For example, the signal input circuit disclosed in Patent Literature 1 includes a charge/discharge section which sets the both ends of an inspection capacitor to values within a fluctuation range. A determination processing section successively performs a charge/discharge procedure which cuts off an inspection path and then charges/discharges the inspection capacitor using the charge/discharge section and a conduction procedure which allows the inspection path to conduct electricity to detect a terminal malfunction.
In the technology in Patent Literature 1, an inspection-dedicated circuit including the inspection capacitor and the charge/discharge section is provided to be connectable or disconnectable to or from a signal line via an inspection switch. As can be presumed from the absence of a sampling capacitor which is charged with a voltage to be subjected to AD conversion in the drawing, it is intended to primarily implement a malfunction detection configuration without any consideration for relation to the AD conversion of an input signal.
When attention is focused on the relations among the respective ON/OFF timings of individual switches, in a 5 V/0 V charging process, a charge/discharge switch is operated to be turned ON/OFF in a state where each of the switch of a multiplexer and the inspection switch is OFF. On the other hand, in a malfunction detection process for an input-side resistor and an input-side capacitor, each of the switch of the multiplexer and the inspection switch is turned ON.
If the inspection switch is connected when the input signal is AD-converted, charge sharing occurs between the inspection capacitor and the sampling capacitor so that an erroneous voltage value different from the voltage value of the input signal is subjected to AD conversion. As a result, a control arithmetic operation using a correct AD conversion value cannot be performed. Even in the case where the inspection capacitor is not particularly provided, when AD conversion is performed in a state where a charge/discharge circuit is connected to the input signal, due to voltage division resulting from an input resistance and the resistance of the charge/discharge circuit, the AD conversion of the erroneous voltage value different from the intrinsic voltage value of the input signal is performed. Consequently, a control arithmetic operation using the correct AD conversion value cannot be performed. This presents a significant problem particularly in a system in which an extremely small error in input value affects steering feeling such as the electric power steering system of a vehicle.
Accordingly, in the technique in Patent Literature 1, it is necessary to independently operate the individual switches such that, when the input signal is AD-converted, the inspection switch is turned OFF and the switch of the multiplexer is turned ON, which results in a complicated switch control configuration. However, depending on a control configuration, it may also be difficult to, e.g., operate the charge/discharge switch independently of the timing of switching the multiplexer.
Also, in the technique in Patent Literature 1, a problem arises in that the inspection switch provided therein results in the need to give consideration to a failure mode in which the inspection switch is fixed in an ON or OFF state, which results in complicated malfunction detection logic.
[Patent Literature 1] JP-A-2012-145410